The invention relates to a process for production of at least one local contact area of a substrate of an electronic component for contact of the area with a connector, wherein the substrate is provided on the contact side with a sintered porous layer that consists of aluminum or contains aluminum.
In semiconductor manufacture, especially production of solar cells, sintered metal contacts are used on the front or rear side of the cell for reasons of production costs.
Usually on the rear side of a silicon solar cell there is a large-area aluminum layer, which, during the manufacture of the solar cell, through heat treatment, is subject to a sintering process, by which simultaneously the rear side of the solar cell is passivated by a so-called back surface field (BSF).
During sintering, the aluminum layer which is in direct contact with the silicon substrate to be designated as the first layer, is melted on at the boundary surface between the aluminum layer and the silicon substrate, and alloyed with the adjoining first layer. During cooling, a silicon layer highly doped with Al congeals in epitaxial fashion on the rear side of the wafer, thus of the substrate. Simultaneous with this, an Al layer enriched with silicon congeals on the Al layer, and at the end of the cooling process, an Al—Si eutectic congeals between the layer highly doped with aluminum and the layer enriched with silicon. The epitaxially grown silicon layer highly doped with aluminum is responsible for the passivation of the rear side of the solar cell. Due to high Al doping, in the semiconductor material of the layer, an excess of negatively charged, fixed location Al acceptors is formed, from which an electric field repulsing the minority carriers is generated, the so-called back surface field.
If the aluminum layer extends over the entire back side of the solar cell or of the substrate, there is, however, a soldering problem, because it is not readily possible directly to solder on tin-plated or non-tin-plated metal connectors, especially copper connectors, on the aluminum back side. To carry out the required electrical contacting in spite of this, normally silver contact strip conductors or soldering points are applied directly onto the substrate surface by screen printing, pad printing or other suitable printing processes and the tin-plated copper bands are soldered onto these. Consequently, in the area of the soldered contact, a recess is provided in the aluminum layer with the result that in this area, no back surface field can form, so that the solar cell rear surface is not completely electrically passivated, and small local photostreams can result.
Because silver is an expensive raw material, it should be dispensed with to reduce manufacturing costs. Therefore it is desirable to avoid the Ag contact altogether.
Direct soldering of the contact bands on the aluminum layer is difficult for several reasons. One reason is the oxidized surface of the Al particles. Another reason is that due to the sintering process, the upper aluminum side is not formed to be sufficiently continuous. Thus, during the sintering process, over the alloy layer doped with Si, an Al layer is produced in the form of spherical Al particles (sintered layer) sintered together from individual particles, in which there is no closed bond of aluminum, but rather a relatively loosely sintered one, which is more or less porous depending on the composition of the aluminum paste or the process parameters during sintering.
If despite this, success were achieved in soldering onto this sintered aluminum layer, nonetheless due to the porosity and the instability of the layer caused by this, only a very slight hold would be obtained. This slight hold reveals itself in small withdrawal forces of about 2-8 N, with the sintered layer torn apart, so that on both sides of the tear location one would detect the spherical structure of the particles.
The same thing happens if the soldered connection on the aluminum layer is subject to tensile forces acting in a module under operating conditions. Small tears can result, causing the soldered location to have little holding capacity, and also could result in higher transitional resistances, or the connection can tear off as a whole and the mechanical and electrical contact is totally destroyed.
From DE-A-10 2007 012 277, a procedure is known for manufacturing a solar cell. In one procedural step, a large aluminum area is applied to the rear side of the semiconductor substrate and is alloyed into it. The aluminum not alloyed into the silicon is removed at least in some areas in an etching step. This requires processing expenses which have a negative impact on manufacturing of solar cells as part of a production line.
EP-A-1 739 690 relates to a solar cell in which, after a back surface field is formed, a previously formed sintered aluminum layer is chemically removed.
According to U.S-A-2003/0108664, to manufacture electrically conducting contacts on solar cells, a precursor mixture is applied. This may contain aluminum. To sinter the material, a pulsed layer beam can be used.
To apply electrical contacts to semiconductor substrates, DE-B-10 2006 040 352 makes a provision to sinter applied metal powder onto the semiconductor substrate by means of a laser beam. Non-sintered material is then removed.
To produce metallic semiconductor contacts according to EP-A-2 003 699, a solderable material is sintered into a sintered aluminum layer applied to a solar cell.